1. Field of the Invention
The present invention relates to a proton conductor, a polymer electrolyte including the same, and a fuel cell employing the polymer electrolyte, and more particularly, to a proton conductor having sufficient ionic conductivity at high temperatures and no humidity, a polymer electrolyte including the same, and a fuel cell employing the polymer electrolyte.
2. Discussion of the Related Art
Fuel cells may be classified according to their electrolyte type. Types of fuel cells include polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and others. The working temperatures of the fuel cells and their constituent materials vary depending on the electrolyte type.
The basic PEMFC may include an anode (fuel electrode), a cathode (oxidizing agent electrode), and a polymer electrolyte membrane interposed between the anode and the cathode. The anode may include a catalyst layer to promote the oxidation of a fuel. The cathode may include a catalyst layer to promote the reduction of an oxidizing agent.
The fuel supplied to the anode may be hydrogen, a hydrogen-containing gas, a mixture of methanol vapor and water vapor, an aqueous methanol solution, or the like. The oxidizing agent supplied to the cathode may be oxygen, an oxygen-containing gas, air, or the like.
Fuel is oxidized to produce protons and electrons at the anode of the PEMFC. The protons migrate to the cathode through an electrolyte membrane and the electrons migrate to an external circuit (load) through a conductive wire (or current collector). The electrons are supplied to the cathode from the external circuit through another conductive wire (or current collector). At the cathode of the PEMFC, the migrated protons react with the electrons and oxygen to produce water. The migration of electrons from the anode to the cathode via the external circuit generates electric power.
In a PEMFC, the polymer electrolyte membrane acts as an ionic conductor for the migration of protons from the anode to the cathode and also acts as a separator to prevent contact between the anode and the cathode. The polymer electrolyte membrane therefore requires sufficient ionic conductivity, electrochemical safety, high mechanical strength, thermal stability at its operating temperature, and should be easily formed into thin layers.
Polymer electrolyte membranes may include a sulfonated perfluorinated polymer with fluorinated alkylene in the backbone and fluorinated vinylether side chains with sulfonic acid at its terminal, such as NAFION, manufactured by DUPONT.
This type of polymer electrolyte membrane may experience a lowered ionic conductivity at operating temperatures of 100° C. or higher due to the loss of moisture by evaporation. It is therefore difficult to operate a PEMFC using this type of polymer electrolyte membrane under atmospheric pressure at about 100° C. or higher. PEMFCs have been operated at 100° C. or lower, for example, at about 80° C.
Methods used to raise the operating temperature of the PEMFC to about 100° C. or higher include a method of providing the PEMFC with a humidification apparatus, a method of operating the PEMFC under pressurize, and a method of using a polymer electrolyte without humidification.
However, when a PEMFC is operated under pressure, the boiling point of water increases, which raises the operating temperature. Furthermore, the use of a pressurizing system or humidification apparatus increases the size and weight of the PEMFC and reduces the overall efficiency of the power generating system. Therefore, a polymer electrolyte membrane with sufficient ionic conductivity at low or no humidity is needed to broaden the range of utilization of the PEMFC.
In conventional fuel cells, water or H3PO4 may be used as the proton conductors. However, when water is used as a proton conductor at high temperatures and no humidity, evaporation may cause a loss of ionic conducting property. When H3PO4 is used as the proton conductor, H3PO4 anions may be adsorbed on the surface of a catalyst, such as Pt, and may deteriorate the performance of the MEA.